Influence of welding parameters on microstructures and mechanical properties of electron beam welded aluminium–lithium plates

Abstract

The present work focused on the welding characteristics of electron beam welding (EBW) in 8090 Al–Li plates, evaluated in terms of strength or toughness degradation in post-weld impact and bending tests with loading rates of 10³, 10^-1, and 10^-4 s^-1. The influence of welding parameters, such as welding power, welding speed, and electron beam focus position, on the post-weld microstructures, porosity, and mechanical properties were examined. Although the joint efficiency for the maximum flexure strength (F_p ), or tensile strength, can be as high as 85–90%, the joint efficiency for the fracture absorption energy (E_t ) was only 20–40%, a level usually unsuitable for applications. It was found that changing the welding power and speed by a factor of 3 resulted in significant variation in E_t but only minor variation in F_p . Changing the welding focus position had little effect on post- weld mechanical properties. The abundant grain boundary precipitates in the welded specimens were thought to be the main cause of the degraded post-weld properties. Other microstructural factors included the δ′ precipitate and grain sizes. The volume fraction of porosity did not play any decisive role owing to the small size (<0·3 mm), low quantity (1–2%), and spherical shape of the EBW induced pores. Finally, given the same post-weld microstructures, the toughness degradation of the EBW specimens was worst under high rate impact loading.     

Controlled surface-flaw-initiated fracture in reaction-bonded Si3N4

Reproducible controlled surface flaws were introduced in reaction-bonded Si₃N₄ by Knoop microhardness indentation. Fracture was initiated at a single suitably oriented flaw on the tensile surface of a four-point bend specimen, and the stress required to propagate the controlled flaw catastrophically was used to calculate the critical stress intensity factor K _IC from standard fracture-mechanics formulae for semi-elliptical surface flaws in bending. K _IC was unchanged up to 1200° C, but values above this temperature together with those at room temperature after vacuum annealing were reduced approximately 25%. This reduction is thought to be due to reaction with oxygen concentrations present in the vacuum atmosphere rather than to a change in material properties. Although flaw healing was observed for annealing in air, it was accompanied by a decrease in fracture stress. It is suggested that this is due to volume mismatch between Si₃N₄ and the oxidation product, cristobalite, which introduces tensile stresses in the matrix material.     

Microstructural characterization of CO2 laser welds in the Al-Li based alloy 8090

The microstructural development of the Al-Li-Cu-Mg-Zr alloy 8090 has been studied after autogenous CO₂ laser welding. Sheets ranging in thickness from 1–4 mm were welded at speeds of between 20–120 mm s^–1 and powers from 1.5–3.8 kW. Optical microscopy, scanning and transmission electron microscopy were used to study the as-received base metal, the heat-affected zone and the solidified fusion zone. The base metal was supplied in a superplastically formable condition and thus had an unrecrystallized grain structure containing 1–2 m sized sub-grains with sub-micrometre and precipitates in the matrix. In the fusion zone, the as-solidified grain structure was columnar at the interface with the base metal but became equiaxed in the central region of the weld pool. The weld depth and top bead width both increased with decreasing welding speed and increasing beam power within the limits investigated. The fusion zone microstructure was cellular-dendritic. Intermetallic precipitates, which are rich in copper, magnesium, silicon (and presumably lithium), formed in the cell/dendrite boundaries. Very fine-scale precipitates were present in the as-solidified -Al matrix but there was no evidence for the , S and T₁ phases. The heat-affected zone was only 100 m wide and was characterized by regions of partial melting. Radiographs of welds reveal that porosity occurred predominantly along the weld centre-line. In partial penetration welds, two types of pores were observed: near spherical and irregular. However, in fully penetrating welds, only the spherical type of porosity was present. Overall volume fractions of porosity were measured from metallographic sections and were found to vary with welding speed and weld type, i.e. partial or full penetration.     

Flow and fracture of a Ni-Fe metallic glass

When brittle failure modes are bypassed, sheet tensile specimens of Ni₄₉Fe₂₉P₁₄B₆Si₂ glass exhibit abrupt shear failures, coincident with yielding; at low to intermediate temperatures no reduction of area is evident in the narrow shear zone. At higher temperatures (up to 0.87 T _g, where T _g is the glass transition temperature) plastic flow at ordinary strain rates is different only in that it is less localized, i.e. yielding induces readily evident necking through the thickness of the sheet; failure still generally occurs by shear rupture through the neck. In the low temperature regime, the yield stress (_y) decreases slowly with increasing temperature; it varies from 3.04 to 1.96 GPa between 77 and 568 K (T/T _g0.82). At higher temperatures, where necking is observed, _y decreases rapidly, apparently approaching zero near T _g (694 K).The oblique shear zones which are generated by yielding apparently follow (or are close to) directions of zero extension (i.e. pure shear) in the sheet specimens. For Ni₄₉Fe₂₉P₁₄B₆Si₂ the angle between the normal to the shear plane and the tensile axis averages 37°, only a few degrees larger than the expected value. This small variation may derive from the dynamics of the yielding process.Oblique necking and shear through the neck occur during tensile deformation of crystalline material only when the sample is in sheet form; also the shear localization zone lies parallel to the thickness vector of the sheet. Metallic glasses are unique in that (1) the shear zone in low aspect ratio sheets is sometimes oblique to the thickness vector (as well as the width vector) and (2) shear yielding also occurs along directions of zero extension in radially symmetric (as opposed to sheet) gauge section specimens.     

Hydrogen impurity in natural gem diamond

Five natural gem diamonds which included type I_A, I_B and II_A specimens, have been investigated by ion beam spectrochemical analysis (IBSCA) to determine the total hydrogen impurity concentration in the diamonds. These data were compared with that obtained from other conventional techniques such as infrared and ultraviolet absorption spectroscopy, electron spin resonance and transmission electron microscopy (TEM). IBSCA indicated that all the specimens appeared to contain 1 at. % of hydrogen contrary to the infrared evidence. The highest hydrogen concentration was recorded by a type I_B specimen which contained small ( 3nm) bubbles. The hydrogen concentration did not correlate with any defect within the specimens, for example the {100} planar faults, and appeared to be a general impurity. No evidence was found to suggest that inclusions were responsible for the impurity.     

Nonisothermal film flow of a magnetizable liquid

The problem of the flow of a nonisothermal magnetizable liquid with a free surface in a nonuniform magnetic field is formulated and investigated theoretically by considering a specific example.Notation H magnetic field intensity - M magnetization - _o magnetic permeability of vacuum - I current (r, , z), cylindrical coordinates - (, gz) coordinates of free surface - R radius of current-carrying conductor - p pressure - v axial component of velocity - viscosity - R₁, R₂ principal radii of curvature of surface - surface tension - Q flow rate of liquid - G characteristic value of gradient of magnetic field intensity - density - g acceleration due to gravity Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 37, No. 5, pp. 881–885, November, 1979.     

Temperature effects on the properties of Ge thin films

The effects of substrate temperature (T_s) on the properties of vacuum evaporated p-type Ge thin films have been investigated for 25s<400°C. Increase in the substrate temperature improves the crystallinity and increases the grain size resulting a gradual change from amorphous to polycrystalline structure which was attained above a substrate temperature of 225°C. Low resistive (1×10^–2 ohm-cm) and high mobility (280 cm²/V·s) films were obtained at T_s=400°C. It has been observed that the conduction mechanism in polycrystalline films was dominated successively by hopping, tunneling and thermionic emission as the sample temperature was increased from 40 to 400 K. In amorphous samples, conduction was described in terms of different hopping mechanisms.     

Thermodynamic Modeling of the Vacuum Synthesis of Transition-Metal Borides

Thermodynamic analysis of the Ti–B–C–O and V–B–C–O systems was carried out with the aim of establishing the conditions of TiB, TiB₂, VB, VB₂, and V₃B₄formation at temperatures from 673 to 1813 K and total pressures in the system from 10⁵to 10^–3Pa. The results indicate that B and B₄C are more suitable boronizing agents than B₂O₃and that the formation of the borides in stoichiometric mixtures is preceded by the formation of carbides and lower borides. The calculations point to a carbon deficiency in the starting mixtures, which can be compensated by introducing free carbon. Preliminary results of electron-beam boronizing demonstrate the feasibility of producing refractory-metal boride coatings on the surface of carbon steels in a vacuum of 10^–2to 10^–3Pa. The thickness of the TiB₂, VB₂, and W₂B₅coatings thus produced is 80–100, 50, and 15–20 m, respectively. A part of the boronizing paste was found to convert into single-phase powder borides.     

Non-crystalline zirconia structure formation by severe mechanical damaging

Zirconia containing 2.85 mol% Y₂O₃ and 1.42 mol% HfO₂ was repeatedly rubbed in one direction without lubricant at room temperature. Transmission electron diffraction (TEM diffraction) taken from the above specimen revealed a structure created as the sum of the contributions of compounds of the composing elements. Further, this also showed halo rings that suggest amorphous zirconium formation. These transformations act as mechanisms of plastic deformation during rubbing.     

Combined weakest link and random defect model for describing strength variability in fibres

A mathematical model which describes the strength variability along the length of a fibre was developed. The model is a combination of the modified weakest link and random defect models. This combined model describes very well the strength variability data of aramid fibres.Nomenclature L Specimen length - F(s) Cumulative frequency distribution of link strengths - 1 — F(s) Survival function of a link - F _L(s) Cumulative frequency distribution of strengths of a specimen of length L - 1 — F _L(s) Survival function of a specimen of length L - s Strength variable - s ₀ Fibre defect-free strength for a random defect or combined model - s ₁, s ₂... Fibre strength at the point of a defect - s ₁, s ₂ ... Strength a fibre must have at the location of the defect to have a strength of s at the location of the defect - Length of a hypothetical link in a weakest link model - ₂, ₂ ... Defect frequencies (mean number per unit length) - v ₁, v ₂ ... Defect severities, 0 v 1 - (s) Defect frequency distribution function defined in terms of the strength at the defect - (v) Defect frequency distribution function defined in terms of the defect severity - , Defect frequency distribution parameters (Equation 14) - a, b Weibull distribution parameters (Equation 4) - P(m) Probability that m defects will occur in a given specimen length - m Number of defects occurring - ¯s Mean strength - CV Coefficient of variation of strength     

DC extended arc plasma nitriding of stainless and high carbon steel

Stainless steel (SS 302) and high carbon steel (HCS) substrates were nitrided in a pot type arc plasma furnace in the temperature range 1100–120 °C under different gas (Ar, N₂, H₂) mixture configurations for twenty minutes each. The nitrided surfaces were characterized by XRD, SEM, metallography and microhardness. Depth of the nitride layer grown was found between 40 to 50 m. Microhardness for SS 302 was observed to increase by about three to four times but for HCS the increase was not more than two times. The major compound phases to grow by this method were indentified to be Fe_2-3N(), (Cr, Fe)N_1-x and CrN in case of SS 302 whereas for HCS the phases were recognised as Fe_2-3N(), (Cr,Fe)N_1-x, Fe₂N() and WN. Further details about the experiment and characterization of arc plasma nitrided steel are reported and discussed in this paper.     

Analysis of the tensile properties and fractography of as-cast and heat treated 359-SiC-20p composite

An experimental study of the heat treatment of 359-SiC 20p composite and its base alloy was made to determine the strength-ductility characteristics under varying conditions of heat treatment. Microstructural observations revealed that addition of the SiC_p reinforcement to the base alloy produced a more uniform and refined interdendritic microstructure compared to the latter. The tensile data obtained was analysed in terms of the theoretical models existing in the composite literature. Ultimate tensile strength (UTS)-log elongation relationships were obtained to test the applicability of the quality index parameter,Q, to the present composite. From this analysis, it was found that all data points in the ageing temperature range 140–210 °C could be represented by a single line (cf. two lines in the case of 359 alloy), indicating the important fact that the tensile properties of this composite can be predicted/determined over the entire temperature range. The presence of the SiC particles was seen to accelerate the Mg₂Si precipitation kinetics, but not to alter it. Fracture mechanisms were determined from both the fracture surfaces and their longitudinal sections beneath the fracture surface, employing both optical and scanning electron microscopy.Nomenclature a Particle diameter - b Burger's vector - b _ii Numerical constant relating P _ii ^E _m andP ₃₃ ^A - E _c Young's modulus of the composite - E _m Young's modulus of the matrix - E _p Young's modulus of SiC particles - El Elongation (%) - f _p SiC volume fraction - P ₃₃ ^A Applied stress - P _ii ^E Long range back stress developed by elastic misfit - P _m ^F Change in matrix flow stress - <P _ii ^P >_m Back stresses due to plastic deformation - P _c ^ps Proof strain of a composite - q _ii Plastic misfit - Q Quality index - R Statistical correlation coefficient - RE Rockwell E hardness value - S SiC particle aspect ratio - S _c Critical aspect ratio for the SiC particles - UTS Ultimate tensile strength of the alloy or composite - YS Yield strength of the alloy or composite - Critical misfit strain - Constant, 1.25 for aluminum alloys - Plastic strain - ^ps Plastic strain at whichP _c ^ps is required - Work hardening rate at a given plastic strain - Work hardening rate as a function of total strain - Shear modulus - Dislocation density - _c ^O Yield stress of the composite - _CTE Increase in yield stress due to coefficient of thermal expansion (CTE) - _m ^O Yield stress of the matrix - _p Particle strength - _i Interfacial shear strength     

The effect of heating on the domain structure of beta-eucryptite,LiAlSiO4

Transmission electron microscopy of -eucryptite revealed the occurrence of antiphase domains with the antiphase vector 1/2 a. This vector relates (Si, Al)O₄-tetrahedra which are chemically equivalent, i.e. either SiO₄ or AlO₄ (in an ordered structure); therefore, it leaves the Si/Al long-range order undisturbed. The size of the domains depends on the temperature history of the crystals. Antiphase domains with the antiphase vector 1/2 c have not been observed. In situ heating experiments showed that the superstructure reflections of the type h+k=2n+1, l=2n+1 (a-reflections) rapidly lose their intensity and become unobservable at temperatures around 550° C (furnace, not specimen temperature) as also known from X-ray studies of -eucryptite at 460° C (specimen temperature). This change is reversible. Correspondingly, the antiphase domain boundaries which are imaged with the a-reflections disappear at these temperatures. Upon cooling, the antiphase domain boundaries reappear at the same positions as before. It is concluded that position, shape and size of the antiphase domains is controlled by the number and distribution of local defects in the Si/Al order.     

Nonlinear elastic analysis of adhesive blister test

A nonlinear elastic analysis of a blister test is conducted for rubber-like material described by the Ogden-Tschoegl strain energy function. A finite element program is applied to the analysis which is formulated based on the total Lagrangian procedure and capable of treating deformation-dependent pressure loading problems. The dependence of specific adhesive fracture energy _a on critical pressure p _cris analyzed for a blister test specimen with different geometry and different material constants. It is suggested that the unbonded radius a in the dimensionless representation p _cr ² a/E_a should be replaced by an average value in the numerical analysis. The numerical analysis of the blister test is also discussed in general.     

Effect of atmosphere on growth of single crystal zinc oxide nanowires

Single crystal zinc oxide (ZnO) nanowires were prepared by using enhanced two-step vapor–liquid–solid (VLS) growth mechanism. The experimental results indicate that the growth rate and morphologies of the nanowires depends on the carrier gas ambient during the thermal reaction process. The ZnO nanowire grown with N₂, not only has the smaller diameter of about 30 nm but also exhibits a higher growth rate and larger number of density of nanowires per unit area than those grown with Ar. The photoluminescence measurements show that the ZnO nanowires grown with N₂ have a stronger ultraviolet emission than those grown with Ar.     

Effects of precracked specimen geometry on local cleavage fracture stress σf of low alloy steel

An elastic-plastic finite element method (FEM) is used to analyze the stress distributions ahead of crack tips for three types of COD specimens with different precracked depth a/W and height W of a low alloy steel, and the tensile and COD tests are carried out at various temperatures. By accurately measuring the distances of the cleavage initiation sites from the blunted crack tips, the local cleavage fracture stresses _f are measured. With increasing precrack depth a/W, specimen height W and test temperatures in a certain range, it was found that the _f essentially does not change. The _f is a steady inherent parameter of the material whose value is independent of the precracked specimen geometry.     

Constraint effects on the ductile-to-brittle transition temperature of ferritic steels: a Weibull stress model

This study examines crack front length and constraint loss effects on cleavage fracture toughness in ferritic steels at temperatures in the ductile-to-brittle transition region. A local approach for fracture at the micro-scale of the material based on the Weibull stress is coupled with very detailed three-dimensional models of deep-notch bend specimens. A new non-dimensional function g(M) derived from the Weibull stress density describes the overall constraint level in a specimen. This function remains identical for all geometrically similar specimens regardless of their absolute sizes, and thus provides a computationally simple approach to construct (three-dimensional) fracture driving force curves _w vs. J, for each absolute size of interest. Proposed modifications of the conventional, two-parameter Weibull stress expression for cumulative failure probability introduce a new threshold parameter _w–min. This parameter has a simple calibration procedure requiring no additional experimental data. The use of a toughness scaling model including _w–min>0 increases the deformation level at which the CVN size specimen loses constraint compared to a 1TSE(B) specimen, which improves the agreement of computational predictions and experimental estimations. Finally the effects of specimen size and constraint loss on the cleavage fracture reference temperature T ₀ as determined using the new standard ASTM E1921 are investigated using Monte Carlo simulation together with the new toughness scaling model.     

Preparation of ceramic coatings from pre-ceramic precursors

Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) has indicated that adherent crack-free coatings of amorphous SiC and Si₃N₄/Si₂N₂O can be built up on planar alumina substrates by pyrolysis of layers of polycarbosilane (PCS) and poly(diphenyl)silazane (PDPS) precursors applied by spin- or dip-coating methods. In general, multilayers of black SiC can be prepared by pyrolysis of PCS layers at 1100°C in a nitrogen atmosphere while transparent coatings consisting of multiple layers of Si₃N₄ are prepared by pyrolysis of either PCS or PDPS layers in a flowing atmosphere of ammonia at 1100°C. The Si₃N₄/Si₂N₂O layers prepared by pyrolysing spin-coated layers of PDPS layers are found to be superior in quality (with respect to blemishes and embedded debris) than those prepared from spin-coated layers of PCS. Microhardness tests reveal that the coatings derived from PCS and PDPS are significantly softer than would be expected for SiC and Si₃N₄. X-ray photoelectron studies reveal that the surface of the PCS-derived SiC coatings consists of an SiO₂ layer while the surface of the PDPS-derived Si₃N₄/Si₂N₂O coating consists of an oxygen-rich silicon oxycarbonitride. These results are also generally supported by Rutherford backscattering spectra which also indicate considerable phase mixing of silicon, carbon, oxygen and nitrogen components within the bulk of the SiC and Si₃N₄/Si₂N₂O coatings on alumina.     

Fine silicon carbide fibers synthesized from polycarbosilane-polyvinylsilane polymer blend using electron beam curing

A fine SiC fiber is synthesized from a polymer blend of polycarbosilane (PCS) and polyvinylsilane (PVS) with electron beam curing under vacuum. The obtained SiC fiber from the PCS-PVS blend polymer has smaller average diameter of 8.5 m than that of 11.8 m from PCS, and shows higher average tensile strength of 3.2 GPa than that of 2.8 GPa from PCS after heat treatment at 1673 K in Ar gas atmosphere. However, the SiC fiber from the polymer blend decreases in tensile strength after heat treatment above 1773 K due to -SiC crystal growth near the fiber surface, because of a small amount of oxygen incorporated in the fiber.     

钛合金薄板激光和钨极氩弧焊残余应力测试研究

采用小孔释放法对钛合金薄板激光焊和钨极氩弧焊(TIG焊)的焊接残余应力进行了测试，并分析了焊接方法、焊接线能量和焊后热处理对残余应力分布规律的影响。研究结果表明：激光焊残余应力分布规律与普通熔焊方法相似，但其分布区域较窄；在热影响区内，激光焊残余拉应力值比TIG焊的约低100MPa；在焊缝及其熔合线附近，激光焊残余应力却比TIG焊的高。对于不同线能量激光焊接，线能量越大，焊缝越宽，热影响区的残余应力也越大。焊后真空热处理能降低残余应力90％。